A selective reduction type NOx catalyst (also referred to hereafter as an SCR catalyst) that selectively reduces NOx contained in exhaust gas when a reducing agent is supplied thereto may be provided in an exhaust passage of an internal combustion engine. The reducing agent supplied to the SCR catalyst is adsorbed to the SCR catalyst. The reducing agent adsorbed on the SCR catalyst is then used to reduce the NOx. Here, a ratio of an amount of NOx reduced by the SCR catalyst relative to an amount of NOx flowing into the SCR catalyst is set as a NOx purification ratio.
Patent Documents 1 and 2 describe an exhaust gas purification apparatus for an internal combustion engine including an SCR catalyst that selectively reduces the NOx contained in the exhaust gas using ammonia (NH3) as the reducing agent. In the exhaust gas purification apparatus described in Patent Documents 1 and 2, ammonia is supplied to the SCR catalyst provided in the exhaust passage by adding urea water to the exhaust gas on an upstream side of the SCR catalyst.
Further, Patent Document 1 discloses a technique of adding at least a prescribed amount of urea water when an estimated value of an amount of ammonia adsorbed on the SCR catalyst is equal to or smaller than a predetermined amount, and adding a smaller amount of urea water than the prescribed amount when the estimated value exceeds the predetermined amount. Here, the predetermined amount is a threshold indicating whether or not at least a fixed NOx purification ratio can be realized and ammonia slippage from the SCR catalyst can be suppressed under all operating conditions of the internal combustion engine.
Furthermore, Patent Document 2 discloses a technique of reducing an amount of urea water supplied to the exhaust gas which increases an amount of ammonia actually adsorbed on the SCR catalyst (the estimated value of the adsorption amount) toward a target adsorption amount as the actual amount of adsorbed ammonia increases toward the target adsorption amount.
In the techniques described in Patent Documents 1 and 2, an amount of ammonia consumed for NOx reduction by the SCR catalyst is calculated on the basis of the amount of NOx flowing into the SCR catalyst and the NOx purification ratio of the NOx catalyst. Further, the amount of ammonia adsorbed on the SCR catalyst is estimated on the basis of an amount of added urea water (an ammonia supply amount) and the amount of ammonia consumed for NOx reduction.